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Abstract

Few birds parasitize living tissue of other birds or mammals. At Golfo San Jose, Argentina, kelp gulls, Larus domincanus, fed on live southern right whale, Eubalaena australis, mothers, landing on their backs and pecking at peeling skin or gouging into the energy-rich hypodermal blubber layer. Most attacks occurred while mothers were resting at the surface in the central nursery area. The typical immediate reaction of mothers to gull attacks was to lower the mid-back and rapidly raise the head and tail stock. Less frequently mothers submerged quickly. After their initial response mothers traveled rapidly, usually underwater, or adopted resting postures that submerged the vulnerable midback area. Gulls attacked singly or in small groups and alternated between following a single pair of whales and attacking several pairs sequentially. The intensity of maternal responses to these attacks affects the energy budget of mothers and calves during a time when energy conservation is important. Subtle postural shifts to submerge the back require less energy than rapid, frantic fleeing. Since gulls feed on whales without causing fatal injury this relationship is termed parasitic.